1. Field of the Invention
The present invention generally relates to a time-division multiplexing communication system, and particularly relates to a time-division multiplexing communication system in which high speed data terminals and low speed data terminals are allowed to exist together with each other and circuit channels can be efficiently allotted to the low speed data terminals.
2. Description of the Prior Art
The improvement in data transmitting efficiency is an important problem to be solved in a communication system which includes a plurality of communication nodes provided on a data transmission line so, that data communication is performed in a time-division multiplexing manner among terminal devices connected to the respective communication nodes. For example, Japanese Patent Unexamined Publication No. 21940/1983 (U.S. Ser. No. 402,118) discloses a time-division multiplexing communication system in which a communication frame transferred on a transmission line is divided into a plurality of time slots and each of the time slots is provided with a validity indication bit for indicating whether the data contained in the time slot concerned is valid or not. In this communication system, if the respective terminal device has data to be sent out and when the terminal device sends out the data into a time slot allotted to the terminal device concerned, the terminal device sets the validity indication bit of the time slot to "1". If the terminal device has no data to be sent out, on the contrary, the terminal device sets the validity indication bit to "0". In the thus arranged communication system, it is sufficient for a terminal device operating as a data receiver to take-in reception data in a time slot as a valid only when the validity indication bit of the time slot is "1", while invalidating the received data when the validity indication bit is "0". Accordingly, a plurality of terminal devices can perform communication by using the same transmission line independently of the data transmitting speed of the respective terminal device. In such a communication system, however, there is a problem that the utilization factor decreases when each of the communication nodes is capable of transmitting data at a high speed because a time slot allotted to a low speed terminal device is occupied for transmission of invalid data. Further the number of the terminal devices which can perform communication at the same time becomes small when the data transmitting capability among the communication nodes is low because a high speed terminal device requires a plurality of time slots to be alotted thereto.
As another conventional communication system, for example, the documents "FUJITSU" published by Fujitsu Ltd., Vol. 33, No. 2 (Pages 153-162) and Vol. 33 No. 3 (Pages 77-86) (1982) describes systems "FOPIC 7300" and "FOPIC 7300AD" in which each time slot is constituted by a train of data bits, a modem control signal, and a parity bit so as to perform inter-terminal communication without providing the validity indication bit. In those systems, there is such a problem that the data transmitting speed of each of the terminal devices should agree with the data transmitting speed among the communication nodes so that the terminal devices which can be connected to a network are limited, because those systems are arranged on the assumption that only valid data are sent by each time slot.